• ru
  • Language: en
  • Documentation version: 2.0

4. How to update denormalized fields in other models on save?

You have models like this.

class Category(models.Model):
    name = models.CharField(max_length=100)
    hero_count = models.PositiveIntegerField()
    villain_count = models.PositiveIntegerField()

    class Meta:
        verbose_name_plural = "Categories"


class Hero(models.Model):
    name = models.CharField(max_length=100)
    category = models.ForeignKey(Category, on_delete=models.CASCADE)
    # ...


class Villain(models.Model):
    name = models.CharField(max_length=100)
    category = models.ForeignKey(Category, on_delete=models.CASCADE)
    # ...

You need the hero_count and villain_count, to be updated when new objects are created.

You can do something like this

class Hero(models.Model):
    # ...

    def save(self, *args, **kwargs):
        if not self.pk:
            Category.objects.filter(pk=self.category_id).update(hero_count=F('hero_count')+1)
        super().save(*args, **kwargs)


class Villain(models.Model):
    # ...

    def save(self, *args, **kwargs):
        if not self.pk:
            Category.objects.filter(pk=self.category_id).update(villain_count=F('villain_count')+1)
        super().save(*args, **kwargs)

Note how we did not use self.category.hero_count += 1, as update will do a DB update.

The alternative method is using signals. You can do it like this.

from django.db.models.signals import pre_save
from django.dispatch import receiver

@receiver(pre_save, sender=Hero, dispatch_uid="update_hero_count")
def update_hero_count(sender, **kwargs):
    hero = kwargs['instance']
    if hero.pk:
        Category.objects.filter(pk=hero.category_id).update(hero_count=F('hero_count')+1)

@receiver(pre_save, sender=Villain, dispatch_uid="update_villain_count")
def update_villain_count(sender, **kwargs):
    villain = kwargs['instance']
    if villain.pk:
        Category.objects.filter(pk=villain.category_id).update(villain_count=F('villain_count')+1)

4.1. Signals vs Overriding .save

Since either of signals of .save can be used for the save behviour, when should you use which one? I follow a simple rule.

  • If your fields depend on a model you control, override .save

  • If your fields depend on a model from a 3rd party app, which you do no control, use signals.

3. How to ensure that only one object can be created?
5. How to perform truncate like operation using Django ORM?